A typical microwave oven door has a metal frame comprising a microwave-blocking screen sandwiched between a pair of transparent panels. As shown in FIG. 3 of U.S. Pat. No. 3,843,859, the exterior transparent panel may be glass and the interior panel may be a relatively thin plastic film such as a polycarbonate film. A pair of epoxy adhesive layers bond the screen to the glass panel and, in turn, the polycarbonate film to the screen. According to column 3, lines 8-12, the adhesive layers can either be coextensive with the screen or only extend around the border. Most microwave oven windows being produced today employ a pair of pressure-sensitive adhesive layers around the border. A few employ mechanical fasteners such as shown in U.S. Pat. No. 3,731,035.
When the interior panel of the window is plastic, it tends to transmit water vapor into the air space between the panels, eventually producing a cloudiness and possibly corroding the microwave-blocking screen. In use, the plastic panel may warp and buckle, further reducing visibility through the window. Since the microwave-blocking screen initially reduces visibility to a near minimum, any significant additional reduction in visibility can require replacement of the window.
A microwave oven window which has no air space at the microwave-blocking screen is disclosed in U.S. Pat. No. 3,431,348. The screen is embedded in silicone resin to provide an elastic inner lamina which is sandwiched between a pair of relatively rigid outer laminae such as glass or acrylic resin sheets. The silicone resin of the inner lamina may be incompletely cured and then further cured in contact with the outer laminae, or the three laminae may be bonded together by separate adhesive layers. The screen extends beyond the composite formed by the three laminae to permit it to make electrically conductive contact with a knitted wire shielding member at the periphery of the window.
We are not aware of any commercialization of the U.S. Pat. No. 3,431,348 construction. The microwave oven windows which we have seen do not provide for electrical contact between the microwave-blocking screen and a separate shielding member. Typically, the microwave-blocking screen and the exterior and interior transparent panels are coextensive, and a microwave-blocking gasket is positioned at the perimeter of the microwave-blocking screen to intercept microwaves escaping around the screen. Often the gasket also serves as a microwave-blocking seal between the oven door and its frame.